The probabilities are more or less reflected in the the current rank
order of the number of OA Eprint Archives of the top 14 countries in
the Institutional Archives Registry:
http://archives.eprints.org/eprints.php?action=browse#country
United States (57),
United Kingdom (33),
Canada (17),
France (15),
Sweden (13),
Germany (12),
Netherlands (12),
Italy (11),
Australia (9),
India (4),
Brazil (4),
Hungary (4),
China (4),
Denmark (3).